Tama Lake 多摩湖 (aka Murayama Reservoir) sits at the edge of Tokyo, next to Sayama Lake 狭山湖 in neighboring Saitama. (And apparently is the homeland of Totoro). Here’s a map that includes both of the lakes:

Photo 1: 水源地を大切に (cherish the water source)

Photo 2: 多摩川の原水の流れ図 (Flow chart of untreated water (gensui) in the Tamagawa River)

Among the many parks in this area is Sayama Park Upper embankment square 狭山公園堤防上広場, at the top of the southern corner of the Murayama-shimo dam 村山下ダム (map). Here’s a photo from the park:

Just downhill from this spot is an old spillway.

Photos: Finished water spillway (from upstream) 完成した余水吐き (上流 から), then and now

Photos: Finished water spillway (from downstream) 完成した 余水吐き (下流 から), then and now (the “now” photo is taken from on top of the bridge seen in the “then” photo)